It is well known that many metallocenes can be useful for the polymerization of olefins. Particularly it has been noted that metallocenes can be combined with aluminoxanes to produce catalyst systems of high activity for the production of polyolefins. A particularly interesting type of metallocene for olefin polymerization is the so-called bridged sandwich-bonded metallocene in which the ligand of the metallocene comprises two cyclopentadienyl-type groups connected through a bridging group. Some of the bridged sandwich-bonded metallocenes when used in the polymerization of olefins having three or more carbon atoms have been found to be useful for producing polymers having different types of microstructure as reflected by tacticity as determined by infrared (IR) and nuclear magnetic resonance (NMR) spectroscopic techniques. A discussion of such techniques is disclosed in Zambelli et al, J. Polym. Sci. Part C, 84, 1488, (1962); Lotz et al, Macro Molecules, 21, 2375, (1988); Haftka et al, J. Macromol. Sci. Phys., 830, 319, (1991); and Youngman et al, Macromol. Res., 2, 33 (1967). A particularly good description of the NMR technique is contained in Chapter 3 of the book "Chain Structure and Conformation of Macromolecules" by Frank A. Bovey (Academic Press, 1982).
It is well known in the art that the microstructure tacticity of a polyolefin molecule can have a significant effect upon the physical properties of the polymer. Other things which affect the polymer properties include the type of monomer, and comonomer if employed, the weight average molecular weight (M.sub.W) of the polymer, the molecular weight distribution, and the composition distribution of the resin. Accordingly, for producing commercially desirable polymers, it is important to find metallocene catalysts which give the desired combination of polymer properties plus commercially practical polymerization activities.
A wide range of sandwich-bonded bridged metallocenes have been at least proposed in the open literature and the patent literature and there have been some studies of the effects of varying the structure of the ligand used in the metallocene. One example of such a study is disclosed in the New Journal of Chemistry, Vol. 14, No. 6-7, pages 499-503 (1990). While the patent literature contains broad assertions regarding the particular types of polymers that will be produced with specific types of metallocenes, subsequent work has revealed that those generalizations are too broad.
For example, while U.S. Pat. Nos. 4,794,096 and 4,769,510 teach that bridged chiral, stereorigid metallocene catalysts are capable of producing polymers having high levels of isotactic microstructure, the only actual examples are ethylene bridged bisindenyl or bis tetrahydroindenyl metallocenes. Further U.S. Pat. No. 4,892,851 shows that the bridged, chiral, sterorigid metallocene cyclopentadienyl isopropylidene fluorenyl zirconium dichloride produces highly syndiotatic polypropylene rather than isotactic polyproplyene. Also, Dr. Abbas Razavi in an SPO 92 paper of September 1992 reported that the bridged, chiral, stereorigid metallocene rac [bis(3-methyl indenyl) ethylene ] zirconium dichloride yields a highly amorphous polyproylene rather than an isotactic polypropylene. Further, while U.S. Pat. No. 4,892,551 contains the broad assertion that bridged metallocenes having two sterically different cyclopentadienyl type groups will produce highly syndiotatic polypropylene, the published EPC Application 423,101 demonstrates that the metallocene 3-methylcyclopentadienyl isopropylidene fluorenyl zirconium dichloride does not produce a syndiotactic polypropylene but rather an amorphous polypropylene referred to as hemiisotactic.
Although the above-mentioned New Journal of Chemistry article discloses two metallocenes having a silyl bridge, it does not contain any information regarding the effects a silyl bridge would have in a metallocene in which the ligand also has a fluorenyl radical. In addition, there are a number of patents which at least envision the possibility of various silyl bridged metallocenes including metallocenes having fluorenyl in the ligand; however, so far as the applicants are aware, there are no publications which discuss the results actually obtained with a silyl bridged sandwich-bonded metallocene which includes fluorenyl in the ligand. Accordingly, there is no actual evidence as to actually what kind of polymer would be produced with the wide range of silyl bridged fluorenyl-containing metallocenes that have been postulated as possible metallocene catalysts.
An object of the present invention is to provide new types of silyl bridged sandwich-bonded metallocene catalysts.
Another object of the present invention is to provide a process for polymerizing olefins using the special silyl bridged metallocenes.
In accordance with yet another aspect of the present invention, there is provided a process for producing novel syndiotactic polymers, particularly syndiotactic polypropylene polymers having unexpectedly low density and low stiffness and tensile properties for their observed degree of syndiotacticity. Still another option of the present invention is to produce homopolymers of propylene having a syndiotacticity as determined by NMR of at least 50% and xylene solubles of at least 50 weight percent, especially those having a syndiotacticity of at least 70% and xylene solubles of at least 60 weight percent, including those having a molecular weight in the range of about 45,000 to 66,000 and a density in the range of about 0.872 to about 0.873. Still another object of the present invention is to provide homopolymers of propylene having syndiotacticity of at least about 70% and an average syndiotactic block length of less than 10, especially those having a syndiotacticity of at least 75% and an average syndiotactic block length of about 9, particularly those having a syndiotactic randomness index of no more than about 0.90, more especially about 0.76. Still yet another object of the present invention is to provide homopolymers of propylene having syndiotacticity of at least 70% wherein the ratio of the average syndiotactic block length to the randomness index is less than 15, more preferably in the range of about 12 to about 13.